The Tampa Bay Times reports the relationship between one-time friends James Pepe and Robert Meredith deteriorated as they worked together at Strawberry Crest High School.
They say 55-year-old Pepe eventually transferred to Bloomingdale High School near Tampa where he was arrested last week.
Plant City police spokesman Tray Towles says Pepe suspected Meredith was spreading rumors about him.
Police started investigating in August after a man called to tell them Pepe tried to hire him to kill Meredith. The caller said he was Pepe's friend.
Pepe remains in Hillsborough County Jail, where he's being held without bond.